Interface control in a bipolar junction transistor

ABSTRACT

Methods of fabricating bipolar junction transistors, bipolar junction transistors, and design structures for a bipolar junction transistor. A first portion of the intrinsic base layer is masked while a second portion of an intrinsic base layer is etched. As a consequence of the masking, the second portion of the intrinsic base layer is thinner than the first portion of the intrinsic base layer. An emitter and an extrinsic base layer are formed in respective contacting relationships with the first and second portions of the intrinsic base layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 13/297,464,filed Nov. 16, 2011, which is hereby incorporated by reference herein inits entirety.

BACKGROUND

The present invention relates to semiconductor device fabrication and,more specifically, to bipolar junction transistors, methods offabricating bipolar junction transistors, and design structures for abipolar junction transistor.

Bipolar junction transistors are typically found in demanding types ofintegrated circuits, especially integrated circuits for high-frequencyapplications. One high-frequency application for bipolar junctiontransistors is in radiofrequency integrated circuits (RFICs), which areused in wireless communications systems, power amplifiers in cellulartelephones, and other types of high speed integrated circuits. Bipolarjunction transistors may also be combined with complementarymetal-oxide-semiconductor (CMOS) field effect transistors in bipolarcomplementary metal-oxide-semiconductor (BiCMOS) integrated circuits,which take advantage of the positive characteristics of both transistortypes in the construction of the integrated circuit.

Conventional bipolar junction transistors are three-terminal electronicdevices that include three semiconductor regions, namely an emitter, abase, and a collector. Generally, a bipolar junction transistor includesa pair of p-n junctions, namely a collector-base junction and anemitter-base junction. A voltage applied across the emitter-basejunction of a bipolar junction transistor controls the movement ofcharge carriers that produce charge flow between the collector andemitter regions of the bipolar junction transistor.

An NPN bipolar junction transistor includes two regions of n-typesemiconductor material constituting the emitter and collector, and aregion of p-type semiconductor material sandwiched between the tworegions of n-type semiconductor material to constitute the base. A PNPbipolar junction transistor has two regions of p-type semiconductormaterial constituting the emitter and collector, and a region of n-typesemiconductor material sandwiched between two regions of p-typesemiconductor material to constitute the base.

Improved bipolar junction transistors, methods of fabricating bipolarjunction transistors, and design structures for bipolar junctiontransistors are needed that advance the capabilities of the technology.

SUMMARY

According to one embodiment of the present invention, a method isprovided for fabricating a bipolar junction transistor. The methodincludes forming an intrinsic base layer and masking a first portion ofthe intrinsic base layer. In response to masking the first portion ofthe intrinsic base layer, a second portion of the intrinsic base layeris etched. The method further includes forming an emitter in acontacting relationship with the first portion of the intrinsic baselayer.

According to another embodiment of the present invention, a bipolarjunction transistor includes an intrinsic base layer with a raisedregion having a first portion and a second portion. The second portionof the raised region is thinner than the first portion of the raisedregion. An emitter is in a contacting relationship with the firstportion of the intrinsic base layer, and an extrinsic base layer is in acontacting relationship with the second portion of the intrinsic baselayer.

According to another embodiment of the present invention, a hardwaredescription language (HDL) design structure is encoded on amachine-readable data storage medium. The HDL design structure compriseselements that, when processed in a computer-aided design system,generates a machine-executable representation of a bipolar junctiontransistor. The HDL design structure includes an intrinsic base layerincluding a raised region with a first portion and a second portion. Thesecond portion of the raised region is thinner than the first portion ofthe raised region. An emitter is in a contacting relationship with thefirst portion of the intrinsic base layer, and an extrinsic base layeris in a contacting relationship with the second portion of the intrinsicbase layer. The HDL design structure may comprise a netlist. The HDLdesign structure may also reside on storage medium as a data format usedfor the exchange of layout data of integrated circuits. The HDL designstructure may reside in a programmable gate array.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate various embodiments of theinvention and, together with a general description of the inventiongiven above and the detailed description of the embodiments given below,serve to explain the embodiments of the invention.

FIGS. 1-8 are cross-sectional views of a portion of a substrate atsuccessive fabrication stages of a processing method for fabricating abipolar junction transistor in accordance with an embodiment of theinvention.

FIG. 4A is a detailed view of the region 4A in FIG. 4.

FIG. 4B is a detailed view of the region 4B in FIG. 4.

FIG. 5A is a detailed view of the region 5A in FIG. 5.

FIG. 5B is a detailed view of the region 5B in FIG. 5.

FIG. 9 is a flow diagram of a design process used in semiconductordesign, manufacture, and/or test.

DETAILED DESCRIPTION

With reference to FIG. 1 and in accordance with an embodiment of theinvention, a substrate 10 includes trench isolation regions 12 whichcircumscribe and electrically isolate a device region 16. The deviceregion 16 is used in the fabrication of a bipolar junction transistor 92(FIG. 8).

The substrate 10 may be any type of suitable bulk substrate comprising asemiconductor material suitable for forming an integrated circuit. Forexample, the substrate 10 may be a wafer comprised of a monocrystallinesilicon-containing material, such as single crystal silicon wafer with a(100) crystal lattice orientation. The monocrystalline semiconductormaterial of the substrate 10 may contain a definite defect concentrationand still be considered single crystal. The semiconductor materialcomprising substrate 10 may include an optional epitaxial layer on abulk substrate, such as an epitaxial layer comprised of lightly-dopedn-type semiconductor material that defines a top surface 25 and thatcovers an oppositely-doped bulk substrate.

The trench isolation regions 12 may be isolation structures formed by ashallow trench isolation (STI) technique that relies on a lithographyand dry etching process to define closed-bottomed trenches in substrate10, fill the trenches with dielectric, and planarize the layer relativeto the top surface 25 of the substrate 10 using a chemical mechanicalpolishing (CMP) process. The dielectric may be comprised of an oxide ofsilicon, such as densified tetraethylorthosilicate (TEOS) deposited bychemical vapor deposition (CVD) or a high-density plasma (HDP) oxidedeposited with plasma assistance.

A collector 18 and subcollector 20 of the bipolar junction transistor 92are present as impurity-doped regions in device region 16. The collector18 and subcollector 20 may be formed beneath the top surface 25 byintroducing an electrically-active dopant, such as an impurity speciesfrom Group V of the Periodic Table (e.g., phosphorus (P), arsenic (As),or antimony (Sb)) effective to impart an n-type conductivity in whichelectrons are the majority carriers and dominate the electricalconductivity of the host semiconductor material. In one embodiment, thecollector 18 and the subcollector 20 may be formed by separate ionimplantations of n-type impurity species and, thereafter, annealing toactivate the impurity species and lessen implantation damage usingtechniques and conditions familiar to one skilled in the art. In aspecific embodiment, the collector 18 may comprise a selectivelyimplanted collector (SIC) formed by implanting an n-type dopant withselected dose and kinetic energy into the central part of the deviceregion 16 and may be formed at any appropriate point in the processflow. In a specific embodiment, the subcollector 20 may be formed by ahigh-current ion implantation followed by lengthy, high temperaturethermal anneal process that dopes a thickness of the substrate 10 beforethe optional epitaxial layer is formed. During process steps subsequentto implantation, the dopant in the collector 18 may diffuse laterallyand vertically such that substantially the entire central portion ofdevice region 16 becomes doped and is structurally and electricallycontinuous with the subcollector 20.

An intrinsic base layer 22, which is comprised of a material suitablefor forming an intrinsic base of the bipolar junction transistor 92, isdeposited as a continuous additive layer on the top surface 25 ofsubstrate 10 and, in particular on the top surface 25 of the deviceregion 16. In the representative embodiment, the intrinsic base layer 22directly contacts the top surface 25 of the device region 16 and a topsurface of the trench isolation regions 12. The intrinsic base layer 22may be comprised of a semiconductor material, such as silicon-germanium(SiGe) including silicon (Si) and germanium (Ge) in an alloy with thesilicon content ranging from 95 atomic percent to 50 atomic percent andthe germanium content ranging from 5 atomic percent to 50 atomicpercent. The germanium content of the intrinsic base layer 22 may beuniform or the germanium content of intrinsic base layer 22 may begraded or stepped across the thickness of intrinsic base layer 22.Alternatively, the intrinsic base layer 22 may be comprised of adifferent semiconductor material, such as silicon (Si). The intrinsicbase layer 22 may be doped with one or more impurity species, such asboron and/or carbon.

Intrinsic base layer 22 may be formed using a low temperature epitaxial(LTE) growth process (typically at a growth temperature ranging from400° C. to 850° C.). The epitaxial growth process is performed after thetrench isolation regions 12 are formed. The epitaxial growth process maybe non-selective as single crystal semiconductor material (e.g., singlecrystal silicon or SiGe) is deposited epitaxially onto any exposedcrystalline surface such as the exposed top surface 25 of device region16, and non-monocrystalline semiconductor material (e.g., polysilicon orpolycrystalline SiGe) is deposited non-epitaxially onto thenon-crystalline material of the trench isolation regions 12 or regions(not shown) where polycrystalline semiconductor material already exists.

The non-selectivity of the growth process causes the intrinsic baselayer 22 to incorporate topography. Specifically, the intrinsic baselayer 22 includes a raised region 24 above the device region 16, anon-raised region 26 surrounding the raised region 24, and a facetregion 28 between the raised region 24 and the non-raised region 26. Theraised region 24 of the intrinsic base layer 22 is comprised ofmonocrystalline semiconductor material and is laterally positioned invertical alignment with the collector 18. A top surface of the raisedregion 24 is elevated relative to a plane containing the top surface 25of the device region 16. The raised region 24 is circumscribed by thetrench isolation regions 12.

The non-raised region 26 of the intrinsic base layer 22 is comprised ofpolycrystalline semiconductor material and overlies the trench isolationregions 12 near the raised region 24. In the absence of epitaxialseeding over the trench isolation regions 12, the non-raised region 26forms with a low growth rate outside of the device region 16. The facetregion 28 of the intrinsic base layer 22 may be comprised of a mixtureof polycrystalline and monocrystalline material or comprised ofprimarily single crystal material in facet region 28. The thickness ofthe intrinsic base layer 22 may range from about 10 nm to about 600 nmwith the largest layer thickness in the raised region 24 and the layerthickness of the non-raised region 26 less than the layer thickness ofthe raised region 24. The layer thicknesses herein are evaluated in adirection normal to the top surface 25 of substrate 10.

A base dielectric layer 32 is formed on a top surface 30 of intrinsicbase layer 22 and, in the representative embodiment, directly contactsthe top surface 30. The base dielectric layer 32 reproduces thetopography of the underlying intrinsic base layer 22 in device region16. The base dielectric layer 32 may be an insulating material with adielectric constant (e.g., a permittivity) characteristic of adielectric. In one embodiment, the base dielectric layer 32 may be ahigh temperature oxide (HTO) deposited using rapid thermal process (RTP)at temperatures of 500° C. or higher, and may be comprised of an oxideof silicon, such as SiO₂ having a nominal dielectric constant of 3.9.Alternatively, if the base dielectric layer 32 is comprised of oxide,the material of base dielectric layer 32 may be deposited by a differentdeposition process, by thermal oxidation of silicon (e.g., oxidation athigh pressure with steam (HIPOX)), or by a combination of oxideformation techniques known to those of ordinary skill in the art.

A sacrificial layer stack 31 including sacrificial layers 36, 40 is thenformed. Sacrificial layer 36 is deposited on a top surface 34 of basedielectric layer 32 and directly contacts the top surface 34.Sacrificial layer 40, which is optional, is deposited on a top surface38 of sacrificial layer 36. The sacrificial layers 36, 40 reproduce thetopography of the underlying intrinsic base layer 22.

Sacrificial layer 36 may be comprised of a material with a differentetching selectivity than the material of the underlying base dielectriclayer 32. In one embodiment, sacrificial layer 36 may be comprised ofpolycrystalline silicon (e.g., polysilicon) deposited by a conventionaldeposition process such as low pressure chemical vapor phase deposition(LPCVD) using either silane or disilane as a silicon source or physicalvapor deposition (PVD). Sacrificial layer 40 may be comprised of adielectric material with a different etching selectivity than thematerial of the underlying sacrificial layer 36. In one embodiment,sacrificial layer 40 may be comprised of Si₃N₄ deposited by CVD oranother suitable deposition process.

With reference to FIG. 2 in which like reference numerals refer to likefeatures in FIG. 1 and at a subsequent fabrication stage, thesacrificial layers 36, 40 of the sacrificial layer stack 31 arepatterned using photolithography and etching processes to definesacrificial mandrels in the form of a sacrificial emitter pedestal 44.To that end, the sacrificial layer stack 31 is masked with a patternedmask layer (not shown). In one embodiment, the mask layer may be aphotoresist layer comprised of a sacrificial organic material applied tothe top surface 42 of sacrificial layer 40 by spin coating andpre-baked. The photolithography process entails exposing the photoresistlayer to radiation imaged through a photomask, baking, and developingthe resultant latent feature pattern in the exposed resist to defineresidual areas of photoresist that mask portions of sacrificial layerstack 31. In particular, the mask includes resist strips coveringrespective surface areas on a top surface 42 of sacrificial layer 40 atthe intended locations of the sacrificial emitter pedestal 44.

An etching process, such as a reactive-ion etching (RIE) process, isused to remove regions of sacrificial layers 36, 40 not protected by themask layer. For example, an initial segment of the etching process mayremove unprotected regions of sacrificial layer 40 and stop on thematerial of sacrificial layer 36. The etch chemistry may be changed toremove unprotected regions of the underlying sacrificial layer 36 andstop on the material of base dielectric layer 32. Alternatively, asimpler etch chemistry might be used that includes fewer etch steps. Atthe conclusion of the etching process, the top surface 34 of basedielectric layer 32 is exposed aside from the portions of the topsurface 34 covered by the sacrificial emitter pedestal 44.

With reference to FIG. 3 in which like reference numerals refer to likefeatures in FIG. 2 and at a subsequent fabrication stage, a hardmasklayer 48 is deposited on a top surface 34 of base dielectric layer 32and directly contacts the top surface 34. The hardmask layer 48 may be aconformal blanket layer with a thickness that is independent of thetopography of underlying features, such as the sacrificial emitterpedestal 44. Hardmask layer 48 may be comprised of a dielectric materialwith a different etching selectivity than the underlying base dielectriclayer 32. In one embodiment, hardmask layer 48 may be comprised ofsilicon nitride (Si₃N₄) deposited using CVD. Alternatively, the materialof hardmask layer 48 may be deposited by another suitable depositionprocess.

After hardmask layer 48 is deposited, a resist layer 50 comprised of aradiation-sensitive organic material is applied to a top surface 49 ofhardmask layer 48 by spin coating, pre-baked, exposed to radiation toimpart a latent image of a pattern including a window 52 to exposesurface areas spatially registered with the device region 16 for bipolarjunction transistor 92, baked, and then developed with a chemicaldeveloper. Window 52 is defined as an opening in the resist layer 50.

A directional anisotropic etching process like RIE that preferentiallyremoves dielectric material from horizontal surfaces, may be used toremove portions of the hardmask layer 48 in regions unmasked by theresist layer 50 to extend the window 52. In particular, an opening withan interior edge 47 is defined in the hardmask layer 48 at the locationof window 52 and extends through the hardmask layer 48 to the topsurface 30 of base dielectric layer 32. In one embodiment, the etchingprocess is selected with an etch chemistry that selectively removesSi₃N₄ in hardmask layer 48 relative to SiO₂ in the base dielectric layer32. The etching process also etches the hardmask layer 48 to formnon-conductive spacers 56 on the sidewalls of the sacrificial emitterpedestal 44. The non-conductive spacers 56 surround the sidewalls of thesacrificial emitter pedestal 44. Following the etching process, theresist layer 50 is removed by oxygen plasma ashing and/or wet chemicalstripping.

With reference to FIG. 4 in which like reference numerals refer to likefeatures in FIG. 3 and at a subsequent fabrication stage, the basedielectric layer 32 is removed by an etching process that stops on thematerial constituting intrinsic base layer 22. At the conclusion of theetching process, the top surface 30 of intrinsic base layer 22 isexposed in device region 16. During the etching process, the patternedhardmask layer 48 operates as a hardmask to selectively mask portions ofbase dielectric layer 32 outside of the device region 16. Thesacrificial emitter pedestal 44 and non-conductive spacers 56 alsorespectively mask surface areas of the base dielectric layer 32 duringthe etching process.

At the conclusion of the etching process, a portion of the top surface30 of intrinsic base layer 22 is exposed between the interior edge 47 ofthe opening in the hardmask layer 48 and the non-conductive spacers 56on the sacrificial emitter pedestal 44. This portion of the top surface30 is an intended location for the extrinsic base layer 64 of thebipolar junction transistor 92.

In one embodiment, the etching process may be chemical oxide removal(COR) that removes the material of base dielectric layer 32, ifcomprised of SiO₂, with minimal undercut beneath the non-conductivespacers 56. A COR process utilizes a vapor or, more preferably, amixture flow of hydrogen fluoride (HF) and ammonia (NH3) in a ratio of1:10 to 10:1 and may be performed at low pressures (e.g., of about 1mTorr to about 100 mTorr) and room temperature. The COR process may beperformed in situ in the deposition chamber or may be performed in anindependent chamber. Sacrificial layer 40 remains unchanged as astructure of the sacrificial layer stack 31 following the etchingprocess. An optional hydrofluoric acid chemical cleaning procedure mayfollow the COR process.

As apparent in FIGS. 4A and 4B, the intrinsic base layer 22 has aninterface layer 60 adjacent to the top surface 30. The interface layer60 represents a thin surface layer that is significantly thinner than abulk layer 21 of the intrinsic base layer 22 and that has an interface62 with the bulk layer 21 of the intrinsic base layer 22. The interfacelayer 60 has a layer thickness, t₁, of the interface layer 60, which ismeasured as a normal distance from the interface 62 to the top surface30. The layer thickness, t₁, in field regions 66 that flank thesacrificial emitter pedestal 44 and non-conductive spacers 56 isnominally equal to the layer thickness, t₁, beneath the sacrificialemitter pedestal 44 and non-conductive spacers 56. In other words, thelayer thickness, t₁, is independent of lateral position proximate todevice region 16.

The interface layer 60 may have a different nominal composition than aregion of the bulk layer intrinsic base layer 22 adjacent to interface62. In one embodiment, a Si seed layer free of Ge may be grown and thenthe intrinsic base layer 22 may be deposited with a graded profile of Geconcentration. In a representative trapezoidal profile, the Geconcentration of the bulk layer 21 is ramped upward from the growthinitiation on the Si seed layer and then fixed at a constant percentageover a plateau within the bulk layer 21 of the intrinsic base layer 22.The Ge concentration of the bulk layer 21 is then ramped downwardly fromthe plateau as the full thickness for the bulk layer 21 is approached.At the interface 62, the downwardly ramped Ge concentration may reachzero so that the interface layer 60 is comprised of intrinsic Si with anegligible Ge concentration. Alternatively, the downwardly ramped Geconcentration may be non-zero near the top surface 30 of the intrinsicbase layer 22 so that the interface layer 60 is SiGe comprised of fivepercent or less of Ge, but still has a lower Ge content than the regionof the bulk layer 21 adjacent to the interface 62. In anotherembodiment, the interface layer 60 may not be initially doped with theimpurity species (e.g., boron and/or carbon) used to dope the bulk layer21 of the intrinsic base layer 22.

With reference to FIG. 5 in which like reference numerals refer to likefeatures in FIG. 4 and at a subsequent fabrication stage, an etchingprocess may be used to reduce the thickness of the interface layer 60 inthe field regions 66, thereby reducing the thickness of the intrinsicbase layer 22 in the field regions 66. If the interface layer 60 iscomprised of Si or primarily of Si, then a RIE process using achlorine-based chemistry (e.g., HCl) may be used to reduce the thicknessof the interface layer 60 in the field regions 66.

In the representative embodiment and as apparent in FIG. 5A, theinterface layer 60 in the field regions 66 has been thinned by theetching process to a nominal layer thickness, t₂, which is measured as anormal distance between a recessed top surface 68 of the intrinsic baselayer 22 and the interface 62 between the interface layer 60 and thebulk layer 21 of the intrinsic base layer 22. The top surface 68 in thefield regions 66 is recessed relative to the top surface 30 beneath thesacrificial emitter pedestal 44 and non-conductive spacers 56 by adistance equal to a thickness, Δ, of interface layer 60 removed by theetching process. The removed thickness, Δ, in the field regions 66 issmaller than the initial layer thickness, t₁, of the interface layer 60and represents the difference between the layer thicknesses t₁ and t₂.If the interface layer 60 is completely removed in the field regions 66,then the removed thickness, Δ, is equal to the initial layer thickness,t₁, and the top surface 30 of the intrinsic base layer 22 is exposed.

As best shown in FIG. 5B, the sacrificial emitter pedestal 44 andnon-conductive spacers 56 respectively mask surface areas of theinterface layer 60 during the etching process. As a result, the initialthickness, t₁, of the interface layer 60 is preserved beneath thesacrificial emitter pedestal 44 and non-conductive spacers 56. Asmentioned above, the difference between the layer thicknesses t₁, t₂ isequal to the removed thickness, A. The result of the etching process isthat the layer thickness, t₂, of the intrinsic base layer 22 differs inthe field regions 66 from the layer thickness, t₁, of the intrinsic baselayer 22 beneath the sacrificial emitter pedestal 44 and non-conductivespacers 56.

In an alternative embodiment, the interface layer 60 may be completelyremoved from the field regions 66. In this instance, the removedthickness, Δ, of the intrinsic base layer 22 will be nominally equal tothe layer thickness t₁ of the interface layer 60. In another alternativeembodiment, the etching process may remove a partial thickness of thebulk layer 21 of the intrinsic base layer 22 in the field regions 66after the interface layer 60 is completely removed from the fieldregions 66. In this instance, the removed thickness, Δ, of the intrinsicbase layer 22 will be greater than the layer thickness t₁.

With reference to FIG. 6 in which like reference numerals refer to likefeatures in FIG. 5 and at a subsequent fabrication stage, an extrinsicbase layer 64 is formed on the recessed top surface 68 of intrinsic baselayer 22 and, in the representative embodiment, directly contacts thetop surface 68. In one embodiment, the extrinsic base layer 64 may becomprised of a semiconductor material (e.g., silicon or SiGe) formed bya selective epitaxial growth (SEG) deposition process. If comprised ofSiGe, the concentration of Ge may have a graded or an abrupt profile ifthe extrinsic base layer 64 is comprised of SiGe, and may includeadditional layers, such as a Si cap. Epitaxial growth is a process bywhich a layer of single-crystal material (extrinsic base layer 64) isdeposited on a single-crystal substrate (intrinsic base layer 22) and inwhich the crystallographic structure of the single-crystal substrate isreproduced in the extrinsic base layer 64. If the chemical compositionof the epitaxial material in the extrinsic base layer 64 differs fromthe chemical composition of the intrinsic base layer 22, then a latticeconstant mismatch may be present between the epitaxial material ofextrinsic base layer 64 and the intrinsic base layer 22.

In an SEG deposition process, nucleation of the constituentsemiconductor material is suppressed on insulators, such as on the topsurface 49 of the hardmask layer 48 and on the non-conductive spacers56. The selectivity of the SEG deposition process forming the extrinsicbase layer 64 may be provided by an etchant, such as hydrogen chloride(HCl), in the reactant stream supplied to the SEG reaction chamber or bythe germanium source, such as germane (GeH₄) or digermane (Ge₂H₆),supplied to the SEG reaction chamber. If the extrinsic base layer 64does not contain germanium, then a separate etchant may be supplied tothe SEG reaction chamber to provide the requisite selectivity. If theextrinsic base layer 64 contains germanium formed using a germaniumsource gas, the provision of an additional etchant to the SEG reactionchamber is optional.

The thinning of the intrinsic base layer 22 in the field regions 66 inpreparation for the growth of extrinsic base layer 64 may be performedwith an etching process in the same tool used to deposit the extrinsicbase layer 64. In this manner, exposure to atmosphere is necessarilyavoided. Alternatively, the substrate 10 may be moved between differentchambers in a tool or between different tools with minimal exposure toatmosphere for sequentially performing the sequential etching anddeposition processes.

The extrinsic base layer 64 may be in situ doped during deposition witha concentration of a dopant, such as an impurity species from Group IIIof the Periodic Table (e.g., boron or indium) effective to impart ap-type conductivity in which holes are the majority carriers anddominate the electrical conductivity of the host semiconductor material.The extrinsic base layer 64 may comprise heavily-doped p-typesemiconductor material. The uneven topography of the underlyingintrinsic base layer 22 might be partially reproduced in the extrinsicbase layer 64 on device region 16 so that the extrinsic base layer 64has a raised region 65 that overlies the raised region 24 of theintrinsic base layer 22.

During the various thermal processes of the process flow, the impurityspecies may be caused to diffuse from the extrinsic base layer 64 intothe intrinsic base layer 22. As a result, the intrinsic base layer 22near the top surface 68 may become doped with an appropriateconcentration of the impurity species so that a low-resistance link tothe intrinsic base layer 22 is formed.

The material in the extrinsic base layer 64 is ultimately used to forman extrinsic base of a bipolar junction transistor 92, which is a NPNbipolar junction transistor in the representative embodiment. During theSEG deposition process, the semiconductor material of the raised region24 and facet region 28 of intrinsic base layer 22 operates as a seedcrystal or crystalline seed that establishes a crystallographic patternfor the semiconductor material of the extrinsic base layer 64 grown onthe raised region 24. The crystallographic pattern of the raised regionof intrinsic base layer 22 is reproduced during selective epitaxy inextrinsic base layer 64 over raised region 24 and facet region 28 sothat this region of the extrinsic base layer 64 has approximately thesame lattice structure and crystalline orientation as intrinsic baselayer 22 taking into account any differences in lattice constant fromdissimilar material compositions.

With reference to FIG. 7 in which like reference numerals refer to likefeatures in FIG. 6 and at a subsequent fabrication stage, an insulatorlayer 72 is deposited that buries the sacrificial emitter pedestal 44.The insulator layer 72 may be comprised of a dielectric, which is aninsulating material having a dielectric constant (e.g., permittivity)characteristic of a dielectric material. In one embodiment, insulatorlayer 72 may be comprised of SiO₂ formed by plasma-enhanced CVD (PECVD)or another suitable deposition process. A top surface 74 of theinsulator layer 72 is planarized using a chemical-mechanical polishing(CMP) process so that the top surface 74 is flat. The CMP processcombines abrasion and dissolution to remove a thickness of the insulatorlayer 72 so that the non-planar topography of the top surface 74 fromthe presence of the sacrificial emitter pedestal 44 is reduced oreliminated, and the top surface 74 is thereby flattened. The CMP processis controlled such that the sacrificial emitter pedestal 44 remainsburied beneath the top surface 74 of the insulator layer 72.

With reference to FIG. 8 in which like reference numerals refer to likefeatures in FIG. 7 and at a subsequent fabrication stage, the topsurface 74 of insulator layer 72 is further recessed relative to thesacrificial emitter pedestal 44 by an etching process, such as RIE.Sacrificial layer 40, which is exposed by the recession of insulatorlayer 72, is then removed from its position between the non-conductivespacers 56. Sacrificial layer 40 may be removed from the sacrificialemitter pedestal 44 using a suitable wet chemical etch or RIE with asuitable etch chemistry.

Sacrificial layer 36, which is exposed after the removal of sacrificiallayer 40, is removed from its position between the non-conductivespacers 56 on the sacrificial emitter pedestal 44. Sacrificial layer 36may be etched using dry etching process that removes the material ofsacrificial layer 36 selective to the materials of base dielectric layer32, non-conductive spacers 56, and base dielectric layer 32. The etchingprocess stops upon reaching the top surface 34 of the base dielectriclayer 32. An etching process such as a hydrofluoric acid type procedurelike a dilute hydrofluoric (DHF) or a buffered hydrofluoric (BHF) wetprocedure, or a COR process is then applied to remove portions of thebase dielectric layer 32 not covered by the non-conductive spacers 56.

As a result of the removal of the sacrificial emitter pedestal 44 frombetween the non-conductive spacers 56, an emitter window 76 is formedbetween the non-conductive spacers 56. The emitter window 76 extendsfrom the apex of the non-conductive spacers 56 to the top surface 30 ofintrinsic base layer 22.

An emitter 78 of the bipolar junction transistor 92 is formed in theemitter window 76. The non-conductive spacers 56 respectively encircleor surround the emitter 78 for electrically isolating the emitter 78from the extrinsic base layer 64. The emitter 78 contacts, and maydirectly contact, the raised region 24 of intrinsic base layer 22 and,therefore, the intrinsic base of bipolar junction transistor 92.

The emitter 78 of the bipolar junction transistor 92 may be formed bydepositing a layer comprised of a heavily-doped semiconductor materialand then patterning the deposited layer using lithography and etchingprocesses. For example, the emitter 78 may be formed from polysilicondeposited by CVD or LPCVD and heavily doped with a concentration of adopant, such as an impurities species from Group V of the Periodic Table(e.g., arsenic) to impart n-type conductivity. The heavy-doping levelmodifies the resistivity of the polysilicon and may be implemented by insitu doping that adds a dopant gas to the CVD reactant gases during thedeposition process.

The lithography process forming the emitter 78 from the layer ofheavily-doped semiconductor material may utilize photoresist andphotolithography to form an etch mask that protects only a strip of theheavily-doped semiconductor material registered with the emitter window76. An etching process that stops on the material of insulator layer 72is selected to shape the emitter 78 from the protected strip ofheavily-doped semiconductor material. The mask is subsequently stripped,which exposes the top surface 74 of insulator layer 72 surrounding theemitter 78.

The insulating layer 70, the extrinsic base layer 64, and the intrinsicbase layer 22 may be patterned using conventional photolithography andetching processes to define an extrinsic base and an intrinsic base ofthe bipolar junction transistor 92. The extrinsic base layer 64 isseparated from the emitter 78 by the non-conductive spacers 56. Sectionsof insulating layer 70 may be retained between the extrinsic base layer64 and the emitter 78.

The emitter 78, intrinsic base layer 22, and collector 18 of the bipolarjunction transistor 92 are vertically arranged. The intrinsic base layer22 is located vertically between the emitter 78 and the collector 18.One p-n junction is defined at the interface between the emitter 78 andthe intrinsic base layer 22. Another p-n junction is defined at theinterface between the collector 18 and the intrinsic base layer 22.

The etching process that provides the differential layer thicknesses forthe intrinsic base layer 22 affords independent, local control over thelayer thickness and, in particular, over the layer thickness of theinterface layer 60. In the field regions 66, the extrinsic base layer 64contacts the top surface 68 of intrinsic base layer 22 to establish anextrinsic base-intrinsic base interface. The deliberate thinning orremoval of the interface layer 60 permits closer spacing between theextrinsic base layer 64 and the bulk layer 21 of the intrinsic baselayer 22. The consequence is that base resistance R_(b) may besignificantly reduced without a significant thermal anneal to drive theimpurity species from the extrinsic base layer 64 to the intrinsic baselayer 22 and thereby dope an adjacent thickness of the intrinsic baselayer 22 to reduce its conductivity and provide a reduced-resistancelink between the intrinsic base layer 22 and extrinsic base layer 64.The base resistance R_(b) is a significant parasitic because it providesan electrical feedback path between the output and input of the bipolarjunction transistor 92. The reduction in the base resistance may improvethe performance of the bipolar junction transistor 92 by increasingspeed of the device, e.g., an important figure of merit, f_(max), whichis a function of base resistance R_(b).

The emitter 78 directly contacts a portion of the intrinsic base layer22 for which the interface layer 60 retains the initial layer thickness,t₁, to establish an emitter-base interface. The maintained thickness forthe interface layer 60 in direct contact with the emitter 78 may beneeded to meet design metrics, such as a specified base-emitter voltageV_(be) at a specified collector current.

During the front-end-of-line (FEOL) portion of the fabrication process,the device structure of the bipolar junction transistor 92 may bereplicated across different portions of the surface area of thesubstrate 10. In BiCMOS integrated circuits, complementarymetal-oxide-semiconductor (CMOS) transistors may be formed using otherregions of the substrate 10. As a result, both bipolar and CMOStransistors available on the same substrate 10.

Standard back-end-of-line (BEOL) processing follows, which includesformation of wiring lines and via plugs in dielectric layers to form aninterconnect structure coupled with the bipolar junction transistor 92,as well as other device structures like bipolar junction transistor 92and optionally CMOS transistors (not shown) included in other circuitryfabricated on the substrate 10. Passive circuit elements, such asdiodes, resistors, capacitors, varactors, and inductors, may befabricated in the interconnect structure and available for use in theBiCMOS integrated circuit.

FIG. 9 shows a block diagram of an exemplary design flow 100 used forexample, in semiconductor IC logic design, simulation, test, layout, andmanufacture. Design flow 100 includes processes, machines and/ormechanisms for processing design structures or devices to generatelogically or otherwise functionally equivalent representations of thedesign structures and/or devices described above and shown in FIG. 8.The design structures processed and/or generated by design flow 100 maybe encoded on machine-readable transmission or storage media to includedata and/or instructions that when executed or otherwise processed on adata processing system generate a logically, structurally, mechanically,or otherwise functionally equivalent representation of hardwarecomponents, circuits, devices, or systems. Machines include, but are notlimited to, any machine used in an IC design process, such as designing,manufacturing, or simulating a circuit, component, device, or system.For example, machines may include: lithography machines, machines and/orequipment for generating masks (e.g., e-beam writers), computers orequipment for simulating design structures, any apparatus used in themanufacturing or test process, or any machines for programmingfunctionally equivalent representations of the design structures intoany medium (e.g., a machine for programming a programmable gate array).

Design flow 100 may vary depending on the type of representation beingdesigned. For example, a design flow 100 for building an applicationspecific IC (ASIC) may differ from a design flow 100 for designing astandard component or from a design flow 100 for instantiating thedesign into a programmable array, for example a programmable gate array(PGA) or a field programmable gate array (FPGA) offered by Altera® Inc.or Xilinx® Inc.

FIG. 9 illustrates multiple such design structures including an inputdesign structure 102 that is preferably processed by a design process104. Design structure 102 may be a logical simulation design structuregenerated and processed by design process 104 to produce a logicallyequivalent functional representation of a hardware device. Designstructure 102 may also or alternatively comprise data and/or programinstructions that when processed by design process 104, generate afunctional representation of the physical structure of a hardwaredevice. Whether representing functional and/or structural designfeatures, design structure 102 may be generated using electroniccomputer-aided design (ECAD) such as implemented by a coredeveloper/designer. When encoded on a machine-readable datatransmission, gate array, or storage medium, design structure 102 may beaccessed and processed by one or more hardware and/or software moduleswithin design process 104 to simulate or otherwise functionallyrepresent an electronic component, circuit, electronic or logic module,apparatus, device, or system such as those shown in FIG. 8. As such,design structure 102 may comprise files or other data structuresincluding human and/or machine-readable source code, compiledstructures, and computer-executable code structures that when processedby a design or simulation data processing system, functionally simulateor otherwise represent circuits or other levels of hardware logicdesign. Such data structures may include hardware-description language(HDL) design entities or other data structures conforming to and/orcompatible with lower-level HDL design languages such as Verilog andVHDL, and/or higher level design languages such as C or C++.

Design process 104 preferably employs and incorporates hardware and/orsoftware modules for synthesizing, translating, or otherwise processinga design/simulation functional equivalent of the components, circuits,devices, or logic structures shown in FIG. 8 to generate a netlist 106which may contain design structures such as design structure 102.Netlist 106 may comprise, for example, compiled or otherwise processeddata structures representing a list of wires, discrete components, logicgates, control circuits, I/O devices, models, etc. that describes theconnections to other elements and circuits in an integrated circuitdesign. Netlist 106 may be synthesized using an iterative process inwhich netlist 106 is resynthesized one or more times depending on designspecifications and parameters for the device. As with other designstructure types described herein, netlist 106 may be recorded on amachine-readable data storage medium or programmed into a programmablegate array. The medium may be a non-volatile storage medium such as amagnetic or optical disk drive, a programmable gate array, a compactflash, or other flash memory. Additionally, or in the alternative, themedium may be a system or cache memory, buffer space, or electrically oroptically conductive devices and materials on which data packets may betransmitted and intermediately stored via the Internet, or othernetworking suitable means.

Design process 104 may include hardware and software modules forprocessing a variety of input data structure types including netlist106. Such data structure types may reside, for example, within libraryelements 108 and include a set of commonly used elements, circuits, anddevices, including models, layouts, and symbolic representations, for agiven manufacturing technology (e.g., different technology nodes, 32 nm,45 nm, 84 nm, etc.). The data structure types may further include designspecifications 110, characterization data 112, verification data 114,design rules 116, and test data files 118 which may include input testpatterns, output test results, and other testing information. Designprocess 104 may further include, for example, standard mechanical designprocesses such as stress analysis, thermal analysis, mechanical eventsimulation, process simulation for operations such as casting, molding,and die press forming, etc. One of ordinary skill in the art ofmechanical design can appreciate the extent of possible mechanicaldesign tools and applications used in design process 104 withoutdeviating from the scope and spirit of the invention. Design process 104may also include modules for performing standard circuit designprocesses such as timing analysis, verification, design rule checking,place and route operations, etc.

Design process 104 employs and incorporates logic and physical designtools such as HDL compilers and simulation model build tools to processdesign structure 102 together with some or all of the depictedsupporting data structures along with any additional mechanical designor data (if applicable), to generate a second design structure 120.Design structure 120 resides on a storage medium or programmable gatearray in a data format used for the exchange of data of mechanicaldevices and structures (e.g., information stored in an IGES, DXF,Parasolid XT, JT, DRG, or any other suitable format for storing orrendering such mechanical design structures). Similar to designstructure 102, design structure 120 preferably comprises one or morefiles, data structures, or other computer-encoded data or instructionsthat reside on transmission or data storage media and that whenprocessed by an ECAD system generate a logically or otherwisefunctionally equivalent form of one or more of the embodiments of theinvention shown in FIG. 8. In one embodiment, design structure 120 maycomprise a compiled, executable HDL simulation model that functionallysimulates the devices shown in FIG. 8.

Design structure 120 may also employ a data format used for the exchangeof layout data of integrated circuits and/or symbolic data format (e.g.,information stored in a GDSII (GDS2), GL1, OASIS, map files, or anyother suitable format for storing such design data structures). Designstructure 120 may comprise information such as, for example, symbolicdata, map files, test data files, design content files, manufacturingdata, layout parameters, wires, levels of metal, vias, shapes, data forrouting through the manufacturing line, and any other data required by amanufacturer or other designer/developer to produce a device orstructure as described above and shown in FIG. 8. Design structure 120may then proceed to a stage 122 where, for example, design structure120: proceeds to tape-out, is released to manufacturing, is released toa mask house, is sent to another design house, is sent back to thecustomer, etc.

The method as described above is used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

References herein to terms such as “vertical”, “horizontal”, etc. aremade by way of example, and not by way of limitation, to establish aframe of reference. The term “horizontal” as used herein is defined as aplane parallel to a conventional plane of a semiconductor substrate,regardless of its actual three-dimensional spatial orientation. The term“vertical” refers to a direction perpendicular to the horizontal, asjust defined. The term “lateral” refers to a dimension within thehorizontal plane.

It will be understood that when an element is described as being“connected” or “coupled” to or with another element, it can be directlyconnected or coupled with the other element or, instead, one or moreintervening elements may be present. In contrast, when an element isdescribed as being “directly connected” or “directly coupled” to anotherelement, there are no intervening elements present. When an element isdescribed as being “indirectly connected” or “indirectly coupled” toanother element, there is at least one intervening element present.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A bipolar junction transistor comprising: anintrinsic base layer including a raised region with a first portion anda second portion, the second portion of the raised region being thinnerthan the first portion of the raised region; an emitter in a contactingrelationship with the first portion of the intrinsic base layer; and anextrinsic base layer in a contacting relationship with the secondportion of the intrinsic base layer.
 2. The bipolar junction transistorof claim 1 wherein the emitter directly contacts the first portion ofthe intrinsic base layer, and the extrinsic base layer directly contactsthe second portion of the intrinsic base layer.
 3. The bipolar junctiontransistor of claim 1 wherein the intrinsic base layer includes a topsurface, a bulk layer, and an interface layer between the bulk layer andthe top surface.
 4. The bipolar junction transistor of claim 3 whereinthe interface layer has a first thickness over the first portion of theintrinsic base layer, the interface layer has a second thickness overthe second portion of the intrinsic base layer, and the second thicknessis smaller than the first thickness.
 5. The bipolar junction transistorof claim 3 wherein the interface layer and the bulk layer are comprisedof silicon-germanium, and the interface layer has a lower germaniumcontent than an adjacent region of the bulk layer.
 6. The bipolarjunction transistor of claim 3 wherein the interface layer is comprisedof silicon, and the bulk layer is comprised of silicon-germanium.
 7. Thebipolar junction transistor of claim 3 wherein the interface layer has adifferent composition than the bulk layer.
 8. The bipolar junctiontransistor of claim 3 wherein the interface layer is comprised ofintrinsic silicon with a negligible germanium concentration, and thebulk layer is comprised of silicon-germanium.
 9. The bipolar junctiontransistor of claim 1 wherein the first portion of the intrinsic baselayer includes a top surface, a bulk layer, and an interface layerbetween the bulk layer and the top surface, the interface layer having adifferent composition than an adjacent region of the bulk layer.
 10. Thebipolar junction transistor of claim 9 wherein the second portion of theintrinsic base layer is free of the interface layer.
 11. The bipolarjunction transistor of claim 1 wherein further comprising: a pluralityof shallow trench isolation regions defining a device region, the deviceregion aligned with the raised region of the intrinsic base layer; and acollector in the device region, the collector comprised of a firstsemiconductor material having an opposite conductivity type from theintrinsic base layer.
 12. The bipolar junction transistor of claim 11wherein the first portion of the intrinsic base layer is disposedbetween the collector and the emitter.
 13. The bipolar junctiontransistor of claim 1 further comprising: a plurality of non-conductivespacers on the emitter, wherein the extrinsic base layer is self-alignedwith the non-conductive spacers.
 14. A hardware description language(HDL) design structure encoded on a machine-readable data storagemedium, the HDL design structure comprising elements that when processedin a computer-aided design system generates a machine-executablerepresentation of a bipolar junction transistor, the HDL designstructure comprising: an intrinsic base layer including a raised regionwith a first portion and a second portion, the second portion of theraised region being thinner than the first portion of the raised region;an emitter in a contacting relationship with the first portion of theintrinsic base layer; and an extrinsic base layer in a contactingrelationship with the second portion of the intrinsic base layer. 15.The HDL design structure of claim 14 wherein the HDL design structurecomprises a netlist.
 16. The HDL design structure of claim 14 whereinthe DHL design structure resides on storage medium as a data format usedfor the exchange of layout data of integrated circuits.
 17. The HDLdesign structure of claim 14 wherein the HDL design structure resides ina programmable gate array.